1. Technical Field
The present invention relates to an overload protector for a mechanical press and more particularly to an overload protector which is used in a mechanical press of multi-point type having a slide connected to a crank shaft through a plurality of connecting rods.
2. Description of Prior Art
There is a conventional overload protector recited in Japanese Utility Model Publication No. 6-18720 as an example of the overload protector of this type. The conventional overload protector is constructed as follows.
Two overload absorbing hydraulic chambers are formed within a slide. The respective hydraulic chambers have pressure receiving members vertically movably inserted thereinto. The pressure receiving members are connected to a crank shaft through connecting rods. The pressure receiving members each has a closing contact portion on its upper end surface. The closing contact portion is brought into closing contact with an under surface of an upper wall of the hydraulic chamber through pressurized oil charged into the hydraulic chamber. When the pressure receiving member descends with respect to the slide by overload imposed during a press working, the closing contact portion opens to relieve the pressurized oil of the hydraulic chamber to an oil reservoir, thereby absorbing the overload.
In order to prevent the leakage of the pressurized oil from the closing contact portion during a normal operation with no overload imposed, the closing contact portion must be precisely machined. However, being provided on the pressure receiving member of a large diameter, the closing contact portion invites a difficulty in handling and requires much labor for its precise machining. Besides, the closing contact portion has to be formed for each of a plurality of pressure receiving members provided in accordance with point number of the mechanical press. This lengthens the time necessary for machining and therefore increases the production cost of the conventional overload protector.
Further, with the conventional overload protector, when overload is imposed on one hydraulic chamber during the press working, the one hydraulic chamber immediately performs an overload operation as mentioned above. On the other hand, the other hydraulic chamber performs an overload operation through a relief valve and a plurality of pipes, which delays its overload operation. As a result, the two hydraulic chambers perform overload operations with a time lag caused therebetween to thereby incline the slide. This entails a likelihood to damage a guiding portion, a driving system or the like of the slide.
The present invention has an object to provide an overload protector which can assure a reliable operation and be manufactured at a low cost.
In order to accomplish the object, the present invention has constructed an overload protector for a mechanical press in the following manner, for example, as shown in FIGS. 1 to 5.
The overload protector comprises a plurality of overload absorbing hydraulic chambers 3a,3b provided within a slide 2 of a mechanical press 1 and a plurality of relief passages 11a,11b communicating the respective hydraulic chambers 3a,3b with an overload protecting valve 12. Check valves 13a,13b and discharge valves 14a,14b are arranged in series with each other in the respective relief passages 11a,11b. The respective check valves 13a,13b inhibit flow from a meeting portion (A) of the relief passages 11a,11b to the respective hydraulic chambers 3a,3b. The respective discharge valves 14a,14b are arranged so as to be able to switch over to a normal condition where they communicate the respective hydraulic chambers 3a,3b with the overload protecting valve 12 and to a discharging condition where they communicate the respective hydraulic chambers 3a,3b with a discharge port (R). When each of the hydraulic chambers 3a,3b has a pressure lower than a set overload pressure, the overload protecting valve 12 is kept closed and the respective discharge valves 14a,14b are held in the normal condition. Conversely, when any one of the hydraulic chambers 3a,3b has a pressure not less than the set overload pressure, the overload protecting valve 12 opens to relieve pressurized oil within the overloaded hydraulic chamber (3a,3b) to an exterior area through flow resistance applying means 78 of the corresponding discharge valve (14a,14b), the meeting portion (A) and the overload protecting valve 12 in order. The discharge valves 14a,14b switch over to the discharging condition based on the fact that the meeting portion (A) reduces its pressure due to flow resistance of the pressurized oil passing through the flow resistance applying means 78.
The present invention operates in the following manner, for example, as shown in FIG. 1 as well as in FIGS. 5(a) to 5(c).
In a state where the slide 2 has returned from a bottom dead center to a top dead center, the hydraulic chambers 3a,3b are charged with pressurized oil of a set charging pressure.
When the slide 2 descends from the top dead center to the bottom dead center and effects a press working of a work in the vicinity of the bottom dead center, a working reaction force increases the pressure of the hydraulic chambers 3a,3b. 
During the press working, with no overload imposed on the respective hydraulic chambers 3a,3b, as shown in FIG. 5(a), pressure ports (Pa),(Pb) each has a pressure which is a normal operation pressure (P0) lower than the set overload pressure. The overload protecting valve 12 is kept closed and the two discharge valves 14a,14b are also closed.
During the press working, when an eccentric working reaction force acts on the slide 2 to increase the pressure of one hydraulic chamber 3a and the pressure port (Pa), the pressurized oil of the thus increased pressure opens one check valve 13a to flow out to the meeting portion (A). However, the other check valve 13b inhibits its flow-out from the meeting portion (A) to the other hydraulic chamber 3b. Conversely, when the eccentric working reaction force increases the pressure of the other hydraulic chamber 3b and the pressure port (Pb), the pressurized oil of the thus increased pressure opens the other check valve 13b to flow out to the meeting portion (A). However, the one check valve 13a prevents its flow-out from the meeting portion (A) to the one hydraulic chamber 3a. 
During the press working, if overload is imposed on one hydraulic chamber 3a for any reason, as shown in FIG. 5(b), one pressure port (Pa) has its pressure increased to an abnormal pressure (P1) not less than the set overload pressure. Then the abnormal pressure (P1) opens the overload protecting valve 12 to discharge the pressurized oil within the one pressure port (Pa) to an exterior area through the flow resistance applying means 78 of the discharge valve 14a, the meeting portion (A) and the overload protecting valve 12. Then the meeting portion (A) rapidly reduces its pressure due to flow resistance of the pressurized oil passing through the flow resistance applying means 78. This enlarges a differential pressure between the respective pressure ports (Pa),(Pb) and the meeting portion (A).
Therefore, as shown in FIG. 5(c), both of the discharge valves 14a and 14b switch over to the discharging condition substantially at the same time, thereby discharging the pressurized oil within the respective hydraulic chambers 3a,3b to the discharge port (R) via the pressure ports (Pa),(Pb) and the discharge valves 14a,14b . This results in allowing the hydraulic chambers 3a,3b to vertically contract and thereby enabling them to absorb the overload.
Also in the event overload is imposed on the other hydraulic chamber 3b, similarly as above, the discharge valves 14b,14a switch over to the discharging condition substantially at the same time to promptly discharge the pressurized oil within the hydraulic chambers 3b,3a. This results in enabling them to absorb the overload.
The present invention produces the following effects.
As mentioned above, the pressurized oil within the hydraulic chambers can be discharged substantially at the same time by switching over the discharge valves to the discharging condition based on a relief operation of the overload protecting valve. Thus it is possible to prevent the inclination of the slide when an eccentric overload is imposed thereon. As a result, this can prevent a guide portion, a driving system or the like of the slide from being damaged.
Differently from the closing contact portion of the above-mentioned conventional overload protector, the overload protecting valve and the discharge valve are satisfactory if each of them has a bore diameter to quickly discharge the pressurized oil of the hydraulic chamber. This can make them compact and easy to handle and reduce the labor for their precise machining, which warrants a sure and highly accurate overload operation. In addition, since it is sufficient if at least one of the overload protecting valve is provided, the overload protector of the present invention is inexpensive, when compared with the conventional one which requires a plurality of closing contact portions.
In consequence, the overload protector of the present invention can assure a reliable operation and be manufactured at a low cost.
Besides, when the slide slightly inclines with an eccentric load imposed thereon while the mechanical press is in normal operation, as mentioned above, the check valve can inhibit the movement of the pressurized oil from a hydraulic chamber which has a high pressure with its pressure increased by the eccentric load, to a hydraulic chamber of a low pressure. This can prevent the slide from further inclining due to pressure increase of the hydraulic chamber of the low pressure.
As a result, the slide experiences only a slight inclination to thereby improve the positioning accuracy at the bottom dead center of the slide. This leads to an increase of the working accuracy.
According to an embodiment of the present invention, the invention is preferably constructed In the following manner, for example, as shown in FIGS. 1 to 5.
Each of the discharge valves 14a,14b comprises a discharge valve seat 71 communicating with any one of the hydraulic chambers 3a,3b, a bypass member 73 which makes an opening and closing movement to the discharge valve seat 71, a resilient means 75 for urging the bypass member 73 to the discharge valve seat 71, a restricting passage 78 provided within the bypass member 73 so as to compose the flow resistance applying means and communicating with the discharge valve seat 71, and an actuation chamber 77 for valve closing which communicates with an outlet of the restricting passage 78 and pressurizes the bypass member 73 for closing. The actuation chamber 77 has a pressurizing sectional area (Y) set to a value larger than that of a sealing sectional area (X) of the discharge valve seat 71.
This embodiment of the invention operates in the following manner, for example, as shown in FIG. 4 as well as in FIGS. 5(a) to 5(c).
As shown in FIGS. 4 and 5(a), in a state where the pressure port (Pa) has a pressure which is the normal operation pressure (P0) lower than the set overload pressure, the pressurized oil within the discharge valve seat 71 produces a valve opening force which is overcome by a force resultant from a pressurizing force for valve closing that the pressurized oil within the actuation chamber 77 for valve closing of the discharge valve 14a produces and an urging force of the resilient means 75 to bring the bypass member 73 into closing contact with the discharge valve seat 71.
As shown in FIG. 5(b), when the pressure port (Pa) has its pressure increased to the abnormal pressure (P1) not less than the set overload pressure, the abnormal pressure (P1) rapidly opens the overload protecting valve 12 to discharge the pressurized oil within the pressure port (Pa) to the exterior area via the restricting passage 78 within the bypass member 73, the actuation chamber 77 for valve closing and the overload protecting valve 12. Simultaneously, the actuation chamber 77 quickly reduces its pressure due to flow resistance of the pressurized oil passing through the restricting passage 78. Accordingly, the valve opening force produced by the pressurized oil within the discharge valve seat 71 becomes larger than the force resultant from the pressurizing force for valve closing produced by the pressurized oil within the actuation chamber 77 and the urging force of the resilient means 75.
The above differential force separates the bypass member 73 from the discharge valve seat 71 to discharge the pressurized oil within the discharge valve seat 71 to the discharge port (R) as shown in FIG. 5(c).
This embodiment of the invention produces the following effect.
The actuation chamber for valve closing reduces its pressurizing force for valve closing interlockingly with the relief operation of the overload protecting valve, thereby immediately separating the bypass member from the discharge valve seat. This can switch over the discharge valve to the discharging condition surely and promptly.
Further, the restricting passage within the bypass member can apply flow resistance to result in the possibility of making the discharge valve compact.
According to another embodiment of the present invention, the invention is preferably constructed in the following manner, for example, as shown in FIG. 4.
Arranged in a radially outer space of the discharge valve seat 71 between an interior area of the discharge valve seat 71 and the discharge port (R) is a fitting wall 80 with which the bypass member 73 fits by a predetermined length at a final time of its closing movement. A fitting portion 80a of the fitting wall 80 defines an inner space which forms a valve-opening holding chamber 81. The valve-opening holding chamber 81 has a pressurizing sectional area (Z) set to a value larger than that of the pressurizing sectional area (Y) of the actuation chamber 77 for valve closing.
This embodiment of the invention operates in the following manner, for example, as shown in FIGS. 5(c) and 5(d).
As shown in FIG. 5(c), rapid separation of the bypass member 73 from the discharge valve seat 71 quickly reduces the pressure of the pressure port (Pa) to thereby start the overload protecting valve 12 closing. Then the actuation chamber 77 has its inner pressure increased to a value near that of an inner pressure of the discharge valve seat 71. The thus increased pressurizing force for valve closing of the pressurized oil within the actuation chamber 77 pushes the bypass member 73 in a closing direction.
However, as shown in FIG. 5(d), just before a leading end of the bypass member 73 starts fitting with a front end of the fitting wall 80, the valve-opening holding chamber 81 has its pressure increased to a value near that of the inner pressure of the discharge valve seat 71. The thus increased inner pressurizing force of the valve-opening holding chamber 81 retains the bypass member 73 separated from the discharge valve seat 71. And the pressurized oil of the pressure port (Pa) is discharged to the discharge port (R) via the interior area of the discharge valve seat 71, the valve-opening holding chamber 81 and the separating gap in order. When the pressure port (Pa) has almost lost its pressure, the urging force of the resilient means 75 brings the bypass member 73 into closing contact with the discharge valve seat 71.
This embodiment of the invention produces the following effect.
The bypass member is pressurized for opening by the pressure of the valve-opening holding chamber once it opens and therefore is kept open irrespective of the overload protecting valve being opened and closed. This can smoothly and quickly discharge the abnormal pressure of the hydraulic chamber without hunting.
According to yet another embodiment of the present invention, the. respective discharge valves 14a,14b and the respective check valves 13a,13b are preferably arranged in order from the respective hydraulic chambers 3a,3b toward the meeting portion (A).
According to this embodiment of the invention, a plurality of check valves can define the meeting portion into a narrow space. This results in decreasing an amount of the pressurized oil residual on an inlet side of the overload protecting valve and therefore enabling the overload protecting valve to perform its operation quickly.
According to yet another embodiment of the present invention, the respective check valves 13a,13b are preferably attached within the bypass members 73,73 of the discharge valves 14a,14b. 
This embodiment of the invention decreases a residual amount of the pressurized oil interposing between the discharge valve and the check valve, thereby switching over the discharge valve promptly and besides making the overload protector compact in its entirety.
According to yet another embodiment of the present invention, the overload protecting valve 12, the discharge valves 14a,14b and the check valves 13a,13b are preferably incorporated Into a common block 36.
This embodiment of the invention decreases a residual amount of the pressurized oil interposing between plural kinds of valves, thereby shortening the operation time of the overload protecting valve and additionally preventing a time lag from occurring in the operation timing of the discharge valve.